1. Field of the Invention
This invention relates generally to two-cycle engines and more particularly to two-cycle engines with means for varying an exhaust port timing in accordance with the engine speed.
2. Prior Art
Generally, engines for vehicles or the like are broadly classified into two types, that is, a low engine speed type designed to achieve good power characteristics at a lower engine speed and a high engine speed type designed to have good power characteristics at a higher engine speed. An engine of the former type achieves the maximum power output in a lower range of the engine speed whereas an engine of the latter type achieves the maximum power output in a higher engine speed. The low speed-type engine fails to output a sufficient power in the higher engine speed range while the high speed-type engine produces a less power output in the lower engine speed range in comparison with the low speed-type. In view of these, it has been proposed to increase the power output in the low engine speed-range without sacrifice of the power output in the high engine speed range, thereby achieving good power output over a wider range of the engine speed. One method of achieving this is to control or vary an exhaust port timing in accordance with the engine speed.
In two-cycle engines, an exhaust port is opened when a piston is moved along a cylinder downwardly, that is, toward its bottom dead center, so that the combustion chamber is communicated with an exhaust passage, thereby discharging the exhaust gas from the combustion chamber. In order to vary the exhaust port timing, it is necessary to vary an effective upper edge of the exhaust port. One example of such two-cycle engines is disclosed in U.S. Pat. No. 4,391,234 and is shown in FIG. 1. More specifically, a recess 130 is formed in an upper wall of an exhaust passage 122 leading to an exhaust port 120 formed in a side wall of a cylinder 116. A drive shaft 134 is disposed at one end of the recess 130 remote from the exhaust port 120 for angular movement about an axis thereof, the drive shaft 134 being disposed substantially perpendicular to both the axis of the cylinder 120 and the axis of the exhaust passage 122. A plate-like valve member 132 is fixedly mounted at one end on the drive shaft 134 so that the valve member 132 is angularly movable about the drive shaft. An end face 132a of the valve member 132 remote from the drive shaft 134 serves as a port timing control surface. The valve member 132 is angularly movable between an open or retracted position where the valve member is received in the recess 134 to open an upper portion of the exhaust port 120 and a closed or extended position where the valve member is extended into the exhaust passage 122 with the control surface 132a closing the upper end of the exhaust port 120, thereby lowering the effective upper edge of the exhaust port to the lower edge of the control surface 132a to delay the exhaust port timing. When the valve member 132 is angularly moved between its open and closed positions, the control surface 132a is moved in an arcuate path. It is necessary that in the closed position of the valve member 132, the control surface 132a should be as close to the inner peripheral surface of the cylinder 116 as possible so as to prevent the combustion gas from leaking between the upper edge of the exhaust port 120 and the control surface 132a of the valve member 132. For this reason, a wall portion 148 defined by the inner peripheral surface of the cylinder 116 and the end face 130a of the recess 130 remote from the drive shaft 134 is thinned into an edge-like shape. This edge-like wall portion 148 has a lowered ability of heat dissipation. Since the edge-like wall portion 148 is disposed at a position where the most elevated temperature develops during the operation of the engine, it has been desired to overcome this problem.
It is considered that in order to overcome the heat dissipation problem, the valve member 132 is thinned, and the recess 130 is correspondingly shallow so that the edge-like wall portion 148 is reduced. With this method, however, when the valve member 132 is brought into its closed or extended position, the thin control surface 132a is completely out of the recess 130, so that there is provided a dead space formed between the recess 130 and the upper surface of the valve member 132. This dead space is in communication with the combustion chamber, and therefore before the exhaust port 120 is opened, part of the exhaust gas flows into the dead space. This would affect the port timing control.